1. Field of the Invention
The present invention relates to an apparatus for projecting a picture of cathode-ray tube, and more paticularly to such an apparatus for correctly projecting a picture displayed on the curved phosphor screen of cathode-ray tube upon a flat plane.
2. Description of the Prior Art
Recently, a technique has been utilized widely in which a picture displayed on a cathode-ray tube (hereinafter referred to as CRT) is projected upon a screen or a recording surface. In general, since the phosphor screen of CRT does not take the form of a flat plane but takes the form of a curved surface (in many applications, a spherical surface), the picture displayed is commensurate with the curved surface. The curved picture is projected upon a flat screen or recording surface. In such a case, the focal point of the optical lens system is matched with respect to the central portion of the picture, and the peripheral portion thereof is not focussed. If the focal point of the optical lens system is matched with respect to the peripheral portion of the picture, the central portion thereof is not focussed. For this reason, with this type of projection apparatus it has been a practice to use a CRT having the phosphor screen whose curved surface approaches as close to a flat plane as possible. However, such an approach is often disadvantageous from technical and economical standpoints of view. More particularly, an enclosure of a CRT which is evacuated at high vacuum degree is required to be strong enough for withstanding atmospheric pressure and in addition, the phosphor screen advantageously has a curved surface since a phosphor material is coated on the CRT panel by precipitating a suspension. In a CRT of 7 inch screen size, for example, the curvature radius of its phosphor screen is considered 1 m at most. Further, if the phosphor screen takes the form of a flat plane, the diameter of a bright spot, which luminesces under bombardment of electron beams, increases at the peripheral portion of CRT screen. This means that a high resolution display cannot be attained over the entire screen. Still further, since the quantity of light is decreased at the peripheral portion of the screen as compared to the central portion thereof, uniform illumination cannot be obtained.